Gutmann–Beckett method - Wikipedia
https://en.wikipedia.org/wiki/Gutmann–Beckett_methodThe Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide (Et 3 PO, TEPO) is used as a probe molecule and systems are evaluated by 31 P-NMR spectroscopy.In 1975, Viktor Gutmann [] used 31 P-NMR spectroscopy to parameterize Lewis acidity of solvents by acceptor numbers (AN).